Reactivity of a Morita–Baylis–Hillman Adduct Derivative Bearing a Triphenylamine Moiety with Lysine Models

The reactivity of Morita-Baylis-Hillman Adduct (MBHA) derivative 7 was studied with different primary amine derivatives such as n-butylamine, Nα-acetyl-L-lysine methyl ester, and a poly-(L-lysine) derivative as lysine models to obtain information about the possible reactions in complex protein environments. MBHA derivative 7 reacted with n-butylamine or Nα-acetyl-L-lysine methyl ester producing monoadducts 9a or 9c, which showed bright emission features in the green region at 526-535 nm with photoluminescence quantum yield values in solutions of 73 % and 51 %, respectively. Based on these results, MBHA derivative 7 can be considered an interesting new fluorogenic probe potentially useful in the labelling of basic amino acid residues. Furthermore, similar to other MBHA derivatives, compound 7 showed the tendency to produce diadducts especially in polar solvents system where specific interactions between the extended aromatic moieties may play a major role

Excitonic Interfacial Proton-Coupled Electron Transfer Mechanism in the Photocatalytic Oxidation of Methanol to Formaldehyde on TiO₂(110)

CH₃OH on a single-crystal rutile TiO₂(110) surface is a widely studied model system for heterogeneous photocatalysis. Using spin-polarized density functional theory with a hybrid functional (HSE06), we study the photocatalytic oxidation of CH₃OH adsorbed at a coordinately unsaturated Ti site as an excited-state process with triplet spin multiplicity. The oxidation to CH₂O is stepwise and involves a CH₃O intermediate. The first O–H dissociation step follows an excitonic interfacial proton-coupled electron transfer mechanism where the hole–electron (<i>h</i>–<i>e</i>) pair generated during the excitation is bound, and the <i>h</i> is transferred to the adsorbate. The O–H dissociation paths associated with other <i>h</i>–<i>e</i> pairs are unreactive, and the moderate experimental efficiency is due to the different reactivity of the <i>h</i>–<i>e</i> pairs. The excited-state CH₃O intermediate further deactivates through a seam of intersection between the ground and excited states. It can follow three different paths, regeneration of adsorbed CH₃OH or formation of the ground-state CH₃O anion or an adsorbed CH₂O radical anion. The third channel corresponds to photochemical CH₂O formation from CH₃OH, where a single photon induces one electron oxidation and transfer of two protons. These results expand the current view on the photocatalysis of CH₃OH on TiO₂(110) by highlighting the role of excitons and showing that adsorbed CH₃OH may also be an active species in the photocatalytic oxidation to CH₂O

Role of Hyperconjugation in the 1,2-Shift Reactivity of Bicyclo[2.1.0]pentane and Cyclopropane Radical Cations: A Computational Study

Hyperconjugation and its relationship with the 1,2-shift rearrangement reactivity in bicyclo[2.1.0]­pentane and cyclopropane radical cations have been studied with density functional theory (PBE0/6-311G**). Hyperconjugation has been evaluated by calculating the ¹H hyperfine coupling constants, atomic spin densities, and dihedral angles of β hydrogens with respect to the axes of the nearest p-orbitals bearing the main part of the localized spin density. The calculated hyperfine couplings are in good agreement with the experimental values, and the calculated couplings and angles satisfy the Heller–McConnell relationship, which validates our approach to measure hyperconjugation. Significantly, it is the endo β-hydrogen on the single methylene bridge of the housanes <b>1a</b>, <b>1b</b>, and <b>1d</b> that has the largest hyperconjugative interaction, and this is also the migrating hydrogen in the 1,2-shift reaction leading to the rearrangement of these housanes to cyclopentene radical cations. As a result of this stereoelectronic preference, the migrating entity from the methylene bridge is the endo rather than the exo bond, irrespective of the nature of the substituent. Accordingly, for the <b>1a</b>–<b>1d</b> housanes, the key role of hyperconjugation lowers the endo C–H or C–Me bond strength selectively, and thereby assists the preferred sigmatropic migration of the endo substituent to the bridgehead carbon. By comparison, the extent of hyperconjugation is found to be much reduced in the cyclopropane radical cations <b>2a</b>–<b>2d</b>, and the latter species do not undergo the corresponding 1,2-shift rearrangement reaction. This absence of reactivity in <b>2a</b>–<b>2d</b> is therefore attributed to the weaker hyperconjugative interaction as well as to the less favorable energetics for the overall reaction

What Controls Photocatalytic Water Oxidation on Rutile TiO₂(110) under Ultra-High-Vacuum Conditions?

The photocatalytic O–H dissociation of water absorbed on a rutile TiO₂(110) surface in ultrahigh vacuum (UHV) is studied with spin-polarized density functional theory and a hybrid exchange-correlation functional (HSE06), treating the excited-state species as excitons with triplet multiplicity. This system is a model for the photocatalytic oxidation of water by TiO₂ in an aqueous medium, which is relevant for the oxygen evolution reaction and photodegradation of organic pollutants. We provide a comprehensive mechanistic picture where the most representative paths correspond to excitonic configurations with the hole located on three- and two-coordinate surface oxygen atoms (O_3s and O_2s). Our picture explains the formation of the species observed experimentally. At near band gap excitation, the O_3s path leads to the generation of hydroxyl anions which diffuse on the surface, without net oxidation. In contrast, free hydroxyl radicals are formed at supra band gap excitation (e.g., 266 nm) from an interfacial exciton that undergoes O–H dissociation. The oxidation efficiency is low because the path associated with the O_2s exciton, which is the most favored one thermodynamically, is unreactive because of a high propensity for charge recombination. Our results are also relevant to understand the reactivity in the liquid phase. We assign the photoluminescence measured for atomically flat TiO₂(110) surfaces in an aqueous medium to the O_3s exciton, in line with the proposal based on experiments, and we have identified a species derived from the O_2s exciton with an activated O_2s–Ti bond that may be relevant in photocatalytic applications in an aqueous medium

Wave Packet Dynamics at an Extended Seam of Conical Intersection: Mechanism of the Light-Induced Wolff Rearrangement

Quantum dynamics calculations on a model surface based on CASPT₂//CASSCF calculations are carried out to probe the traversal of a wave packet through an extended seam of conical intersection during the light-induced Wolff rearrangement of diazonaphtoquinone. The reaction is applied in the fabrication of integrated circuits. It consists of nitrogen elimination and ring rearrangement to yield a ketene. After excitation, the wave packet relaxes and reaches the extended seam. A fraction of the wave packet decays to the ground state at a region of the seam connected to a carbene intermediate, while the remaining part decays at a region leading to the ketene. The passage of the wave packet through the extended seam explains the competition between concerted ketene formation and a stepwise mechanism involving a carbene. The two primary photoproducts are formed in the first 100 fs of the simulation, in agreement with recent ultrafast spectroscopy measurements

Conical Intersection Optimization Using Composed Steps Inside the ONIOM(QM:MM) Scheme: CASSCF:UFF Implementation with Microiterations

Three algorithms for optimization of minimum energy conical intersections (MECI) are implemented inside an ONIOM­(QM:MM) scheme combined with microiterations. The algorithms follow the composed gradient (CG), composed gradient–composed steps (CG-CS), and double Newton–Raphson-composed step (DNR-CS) schemes developed previously for purely QM optimizations. The CASSCF and UFF methods are employed for the QM and MM calculations, respectively. Conical intersections are essential to describe excited state processes in chemistry, including biological systems or functional molecules, and our approach is suitable for large molecules or systems where the excitation is well localized on a fragment that can be treated at the CASSCF level. The algorithms are tested on a set of 14 large hydrocarbons composed of a medium-sized chromophore (fulvene, benzene, butadiene, and hexatriene) derivatized with alkyl substituents. Thanks to the microiteration technique, the number of steps required to optimize the MECI of the large molecules is similar to the one needed to optimize the unsubstituted chromophores at the QM level. The three tested algorithms have a similar performance, although the CG-CS implementation is the most efficient one on average. The implementation can be straightforwardly applied to ONIOM­(QM:QM) schemes, and its potential is further demonstrated locating the MECI of diphenyl dibenzofulvene (DPDBF) in its crystal, which is relevant for the aggregation induced emission (AIE) of this molecule. A cluster of 12 molecules (528 atoms) is relaxed during the MECI optimization, with one molecule treated at the QM level. Our results confirm the mechanistic picture that AIE in DPDBF is due to the packing of the molecules in the crystal. Even when the molecules surrounding the excited molecule are allowed to relax, the rotation of the bulky substituents is hindered, and the conical intersection responsible for radiationless decay in solution is not accessible energetically

Conical Intersection Optimization Based on a Double Newton–Raphson Algorithm Using Composed Steps

An algorithm for conical intersection optimization based on a double Newton–Raphson step (DNR) has been implemented and tested in 11 cases using CASSCF as the electronic structure method. The optimization is carried out in redundant coordinates, and the steps are the sum of two independent Newton–Raphson steps. The first step is carried out to reach the energy degeneracy and uses the gradient of the energy difference between the crossing states and the so-called branching space Hessian. The second step minimizes the energy in the intersection space and uses the projected excited state gradient and the intersection space Hessian. The branching and intersection space Hessians are obtained with a Broyden–Fletcher–Goldfarb–Shanno update from the gradient difference and projected excited state gradients, respectively. In some cases, mixing of the quasi-degenerate states near the seam causes changes in the direction of the gradient difference vector and induces a loss of the degeneracy. This behavior is avoided switching to a composed step (CS) algorithm [Sicilia et al.<i> J. Chem. Theory Comput.</i> <b>2008</b>, <i>4</i>, 27], i.e., a hybrid DNR-CS implementation. Compared to the composed gradient (CG) [Bearpark et al. <i>Chem. Phys. Lett.</i> <b>1994</b>, <i>223</i>, 269] and hybrid CG-CS algorithms, the DNR-CS algorithm reaches the MECI in 30% and 15% less steps, respectively. The improvement occurs mostly because the approach to the seam is more efficient, and a degeneracy threshold of 0.001 hartree is reached at lower energies than in the CG and CG-CS cases

Triplet Mediated C–N Dissociation versus Internal Conversion in Electronically Excited <i>N</i>‑Methylpyrrole

The photochemical and photophysical pathways operative in <i>N</i>-methylpyrrole, after excitation in the near part of its ultraviolet absorption spectrum, have been investigated by the combination of time-resolved total ion yield and photoelectron spectroscopies with high-level ab initio calculations. The results collected are remarkably different from the observations made for pyrrole and other aromatic systems, whose dynamics is dictated by the presence of πσ* excitations on X–H (X: N, O, S, ...) bonds. The presence of a barrier along the C–N dissociation coordinate that can not be tunneled triggers two alternative decay mechanisms for the S₁ A″ πσ* state. While at low vibrational content the C–N dissociation occurs on the surface of a lower ³ππ* state reached after efficient intersystem crossing, at higher excitation energies, the A″ πσ* directly internally converts to the ground state through a ring-twisted S₁/S₀ conical intersection. The findings explain previous observations on the molecule and may be relevant for more complex systems containing similar C–N bonds, such as the DNA nucleotides

Synthesis and Isomeric Analysis of Ru^II Complexes Bearing Pentadentate Scaffolds

A Ru^II-pentadentate polypyridyl complex [Ru^II(κ-N⁵-bpy2PYMe)­Cl]⁺ (<b>1</b>⁺, bpy2PYMe = 1-(2-pyridyl)-1,1-bis­(6–2,2′-bipyridyl)­ethane) and its aqua derivative [Ru^II(κ-N⁵-bpy2PYMe)­(H₂O)]²⁺ (<b>2</b>²⁺) were synthesized and characterized by experimental and computational methods. In MeOH, <b>1</b>⁺ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, <i>trans</i>-<b>1</b>²⁺ generates a [Ru^III(κ-N⁴-bpy2PYMe)­Cl₂]⁺ intermediate after the exchange of a pyridyl ligand by a Cl^– counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru­(II). On the contrary, <i>cis</i>-<b>1</b>²⁺ is in direct equilibrium with <i>trans</i>-<b>1</b>²⁺, with absence of the κ-N⁴-bis-chlorido Ru^III-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure <i>trans</i>-[Ru^II(κ-N⁵-bpy2PYMe)­(H₂O)]²⁺ isomer (<i>trans-</i><b>2</b>²⁺), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [Ru^II(κ-N⁵-bpy­(bpyMe)­PYMe)­Cl]⁺ (<b>3</b>⁺) and [Ru^II(κ-N⁵-bpy­(bpyMe)­PYMe)­(H₂O)]²⁺ (<b>4</b>²⁺) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [Ru^II(PY5Me₂)­X]^<i>n</i>+ system (X = Cl, <i>n</i> = 1 (<b>5</b>⁺); X = H₂O, <i>n</i> = 2 (<b>6</b>²⁺)), which also contains a κ-N⁵–Ru^II coordination environment, and to the newly synthesized [Ru^II(PY4Im)­X]^<i>n</i>+ complexes (X = Cl, <i>n</i> = 1 (<b>7</b>⁺); X = H₂O, <i>n</i> = 2 (<b>8</b>²⁺)), which possess an electron-rich κ-N⁴C–Ru^II site due to the replacement of a pyridyl group by an imidazolic carbene